The present invention relates generally to polymers and more particularly to strong liquid-crystalline polymers.
Liquid crystals (LCs) are phases with partial molecular ordering intermediate between the random packing (disorder) of a liquid, and the regular three-dimensional packing (order) of a crystal. In the nematic LC phase, there is partial orientational ordering of the long axes of the molecules; in the smectic LC phases, there is partial orientational ordering (as in the nematic phase) and also partial positional ordering of the centers of mass of the molecules into layers. Liquid-crystal polymers (LCPs) are polymers that form solids by cooling from a LC phase (either melt or solution), rather than from an isotropic liquid phase (melt or solution) as is the situation with non-liquid-crystalline polymers. In general, the greater the crystalline order of the polymer, the greater the strength. More specifically, increasing the orientational ordering of the polymer molecules increases both the tensile strength and the tensile modulus of the polymer. (See, e.g., Elasticity, Plasticity, and Structure of Matter, third edition, edited by R. Houwink and H. K. de Decker, Cambridge at the University Press, 1979.) Certain solidified liquid-crystal polymers are used as stronger, lighter-weight replacements for metals, ceramics and other materials in various structural applications including auto and airplane parts, armor, belted tire cord, sails, mooring lines, etc. Advantages of solidified LCPs as compared with metals, ceramics, and other structural materials include low density (1-2 g/cm.sup.3 versus about 7.8 g/cm.sup.3 for steel). Moreover, LCPs can be synthesized in a wide variety of chemical structures permitting properties to be tailored to particular requirements.
The tensile strength of polymeric materials is in part due to the chemical bonding of atoms in very long chains (polymer molecules) in contrast to the weaker interactions (physical or metallic attractions between atoms) in other materials, such as metals. Solidification of polymers from a solution or a melt virtually always occurs more rapidly than the polymer molecules can arrange themselves in a regular ordered array characteristic of perfect crystalline order. Therefore, solid polymeric materials have significantly less than perfect crystalline order, especially in sample sizes suitable for applications. However, molecular ordering (and thereby strength) of a solid polymer can be increased by forming the solid by cooling from a phase with some orientational order already present (i.e., a LC phase). The partial disorder of this LC phase is important in that the LC phase has sufficient fluidity so that the polymer can be physically processed (formed, spun, etc.) before solidification.
Existing LCPs with structural uses are all backbone LCPs in which main chains (backbones) of the polymer molecules have a number of linearly connected, relatively rigid sections (formed by LC chemical structures having low molecular weight). The chains will pack together more efficiently if they align in such a manner that their backbones are parallel to each other. The rigid sections are rod- or disc-shaped sections having their long axes in the long axis of the backbone of the polymer. There can be semiflexible sections in the backbones, but rigid sections provide superior mechanical properties. The other general class of LCPs are the side-chain LCPs. Therein, the main-chain backbones are flexible (as in non-liquid-crystalline polymeric materials such as polyethylene), but the pendant side groups attached thereto have rod or disc-shaped rigid sections (again formed by small-molecule LC chemical structures) forming an overall structure having the general appearance of a comb. Such structures may pack together more efficiently if the side chains on neighboring backbones align and intermesh with each other in layers.
Present backbone LCPs have good mechanical properties (such as tensile strength and tensile modulus) parallel to the long axes of the backbones, but have poor mechanical properties (such as compressive strength) in the transverse direction. High-strength fibers consistently exhibit a tendency to fibrillate (i.e., individual strands in the fiber bundle separate). Therefore, current backbone LCPs are limited to superior mechanical properties in one dimension and are thereby limited in technical applications principally to fibers in woven fabrics and in composite matrices.
Several laboratories have prepared and investigated combined LCPs; that is, those with liquid-crystalline backbones and liquid-crystalline side chains. Most notably, the journal articles entitled "Combined Liquid Crystalline Polymers: Mesogens In The Main Chain And As Side Groups," by Bernd Reck and Helmut Ringsdorf, Makromol. Chem., Rapid Commun. 6, 291 (1985), "Structural Variations of Liquid-Crystalline Polymers: Cross-Shaped And Laterally Linked Mesogens In Main Chain And Side Group Polymers," by Sibylle Berg, Volker Krone, and Helmut Ringsdorf, Makromol. Chem., Rapid Commun., 7, 381 (1986), and "Combined Liquid-Crystalline Polymers: Rigid Rod And Semiflexible Main Chain Polyesters With Lateral Mesogenic Groups," by Bernd Reck and Helmut Ringsdorf, Makromol. Chem., Rapid Commun. 7, 389 (1986) all describe combined liquid-crystalline polymers having a wide variety of structures, but no investigation of the strength parameters of the generated compounds is mentioned. In fact, although the authors state in the first reference (page 297, lines 1-2) that a biaxial orientation has been observed for the species investigated, on page 393 of the third article, the authors show a figure (C) which is meant to explain the type of interaction which may lead to observed smectic phases. The long axes of the side chains associated with the backbone of one molecule align parallel to the long axes of the backbones of the molecules, thereby yielding a uniaxial orientation of the molecules. That is, the authors explain the observed bidirectional X-ray diffraction pattern for melt-drawn fibers as likely being the result of the existence of two orientations of uniaxial domains within the cross section of the investigated fibers. As will become clear hereinbelow, the Ringsdorf materials utilize side chains which are too flexible and/or have too great a spacing to provide significant increases in structural strength.
By contrast, in "Phenylbenzthiazole Pendent p-Terphenylene Rigid-Rod Benzobisazole Polymers," by J. Burkett and F. E. Arnold, Am. Chem. Soc. Div. Polym. Preprints 28, (2) 278 (1987) and in "Heterocyclic Pendant Rigid-Rod Lyotropic Liquid Crystalline Polymers," by Tsu-tzu Tsai and Fred E. Arnold, Am. Chem. Soc. Div. Polym. Preprints 27, (2) 221 (1986), the authors set out to improve solubility and compressive properties of rigid-rod polymers by structural modifications of the backbone. Pendant groups were utilized to prevent the close packing of the rods, thereby promoting solubility in solvents other than strong acids. It is reported that modulus and tenacity values are lower for pendant systems when compared with non-pendant systems, while compressive strain properties improved. However, in both references, Arnold discloses that the long pendant groups prevent the alignment of polymer chains in solution, thereby improving solubility. Therefore, since structural strength is related to the degree of ordering of the polymers, the Arnold articles teach away from the generation of strong materials. Moreover, in a similar manner to Ringsdorf, Arnold teaches side-chain spacing which is considerably larger than that which would lead to significant increases in material strength.
Several articles by M. Ballauff et al. describe the placement of flexible side chains onto rigid rod polymers. For example, in "Rigid Rod Polymers With Flexible Side Chains, 2. Observation of A Novel Type Of Layered Mesophase," by M. Ballauff and Gunter Friedrich Schmidt, Makromol. Chem., Rapid Commun. 8, 93 (1987), the authors observed, using x-ray diffraction, what they believe is layer formation for polymers having side chains where the number of carbons exceeds eight. They explain this effect by intercalation of side chains as shown in FIG. 3 of the paper. However, the ordering of these side chains, as determined by x-ray diffraction, is weak and short-range since side chains employed are too flexible to order well, and thus to give good compressive strength.
In "New Theories For Smectic And Nematic Liquid-Crystal Polymers: Backbone LCPs And Their Mixtures And Side-Chain LCPs," by F. Dowell, Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157, 203 (1988), the author developed a first-principles, microscopic, molecular statistical-physics theory to predict and explain static thermodynamic and molecular ordering properties of LCPs in various liquid-crystalline phases and in the isotropic liquid phase. These theories can be used to explain the packing (ordering) of the molecules from the specific chemical structure of the molecules. The theories include segmental intermolecular interactions such as hard repulsions (which influence steric packing), soft repulsions, London dispersion attractions, dipole/dipole forces, dipole/induced-dipole forces, and hydrogen bonding arising on polymer molecules composed of various explicit rigid rodlike and explicit semiflexible chain sections in backbones or in side chains. Thermodynamic properties [the temperature range of practical uses for LCPs depends upon the thermodynamic properties (including phase stabilities)] and molecular ordering properties can be calculated as a function of temperature, pressure, degree of polymerization, and chemical structure in the LCP molecules. There are no ad hoc or arbitrarily adjustable parameters.
The theory developed in this paper and in earlier papers by Dowell as referenced in this paper have been applied to the identification of molecular characteristics which optimize the two partially conflicting requirements for strong LCPs: namely, molecular order and fluidity. Molecules are chosen by experience and intuition, and the calculations provide the predicted properties. Important characteristics have been found to include the length and shape of the side chains and the backbone repeat units, the number of side chains per repeat unit, the spacing of the side chains along the backbone, the flexibility of the backbone sections and the side-chain sections, and the use of a flexible connecting group between each side chain and the backbone. Specific inputs to the calculations include bond lengths and angles, chain internal trans-gauche rotation energies, site-site Lennard-Jones pair potentials, dipole moments, polarizabilities, hydrogen bonding, etc. for individual atoms and small groups of atoms such as benzene rings and methylene groups in the LCP molecules. Solvent effects can also be predicted and explained utilizing the same approach. (For practical applications, some strong LCPs will require processing from solution.) The calculations have been found to be in good quantitative agreement with existing thermodynamic and molecular order data for known LCPs.
Accordingly, it is an object of the present invention to set forth the significant parameters for the design of liquid-crystalline polymeric compositions of matter having better mechanical properties than existing LCPs.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.